As is known in the art, it is frequently required to make electrical contact from the upper surface of a multi-level printed circuit board to underlying electrical conductors within the board such as shown in FIG. 1A. Such connections are typically made by drilling a hole from the upper surface into the printed circuit board with such hole cutting through the desired underlying as shown in FIG. 1B. The walls of the hole are, after cleaning, coated with an electrically conductive material thereby forming an interstitial via, as shown in FIG. 1C.
Military products have often been processed with extreme positive or negative etch back features. The positive feature is produced by removing resin from the hole wall until the inner-layer copper foil (i.e., the underlying electrical conductor) protrudes as a tongue into the hole. Electro-less and anodic copper is then plated to the resin barrel on the via and also over-plates the protruding (so called positive) tongue. Negative etch back is produced by etching inner-layer copper foil away from the hole wall so that the edges of the foil are recessed inwardly from the walls of the drilled hole as shown in FIG. 2A. The electroless and anodic copper then penetrate the circumferential etch recess or groove, associated with each inner-layer foil plane, thus making a recessed interconnect as shown in FIG. 2B. Military standards also require rectangular interface structures with minimal copper wedge at the plane to via barrel transition. Positive and Negative Etch Back features are not typically found in commercial products. This is due to the yield and cost burden associated with the additional process requirements.